1. Field of the Invention
The present invention relates to a device for generating a compensation loop on a web that is sent to graphic machines, such as printers.
2. Description of Related Art
In the graphic industry, there is often a problem of problem of providing a continuous web of printing paper coming from a supply, such as a reel or a box containing the web in a folded configuration, to a printer, wherein the printer may work in a intermittent way, while the web, even though it does not have to arrive in a continuous manner and at a constant rate to the printer, however it cannot be suddenly started or stopped in its running because it can be torn or jammed in an excessive quantity at the printer input, where sliding problems might arise.
Therefore, it is an object of the present invention to provide a device generating a loop in a web feeding a graphic machine, for compensating the intermittent operation of the graphic machine, because sometimes the web is not required and sometimes there is a strong necessity therefor.
Another object is to provide a signal capable of detecting the formation of a loop in a web at the input of the graphic machine. Another object is to stop the web letting in to the graphic machine when a loop formed in the device exceeds a first prefixed length and to start it again when the loop is shorter than a second prefixed length (that is its length is too short). A further object is to prevent the paper from being spoiled by causing stopping or starting of the movement of the web to take place in a gradual way.
A known method to signal an excessive shortening or lengthening of the loop is to employ very sensitive microswitches receptively connected both to a tray whereon the loop bottom can lay and to a short rod that can be lifted from the loop when the loop is shortening too much. The microswitches operate a motor controlling the entrance of the web to the graphic machine.
This method has the disadvantage of requiring very sensitive microswitches and consequently, the machine is readily disturbed by vibrations typical of these kinds of machines. Moreover, very weak microswitches can fail very often losing their operating and therefore the control of the web loop with consequences that can be easily understood.
Another method uses proximity detectors such as capacity detectors by which the capacity variation of an electrode as caused by the presence of an insulating material such as a web is sensed. These kind of detectors have a good strength and are relatively insensitive to vibrations, but they have a drawback of requiring frequent adjustments when the web composition changes or the environmental conditions change the moisture content of said web.